As a transmission for a vehicle, in recent years, a so-called dual clutch type transmission is known. In order to avoid an interruption of mechanical power transmission at a time of shifting, the dual clutch type transmission includes a first clutch and a second clutch. The first clutch can engage between an input shaft of a first shifting mechanism structured by odd number gear positions (hereinafter referred to as “first input shaft”) and an output shaft of an internal combustion engine (hereinafter referred to as “engine output shaft”). The second clutch can engage between an input shaft of a second shifting mechanism structured by even number gear positions (hereinafter referred to as “second input shaft”) and the engine output shaft. The dual clutch type transmission changes gears by alternately engaging these two clutches. When shifting from one of the odd number gear positions to one of the even number gear positions, for example, the dual clutch type transmission releases the first clutch for transmitting a mechanical power to the even number gear position and engages the second clutch for transmitting the mechanical power to the even gear position of which a gear pair is beforehand in mesh. Thus, the dual clutch type transmission avoids the interruption of power transmission at the time of shifting.
Further, Patent document 1 discloses a hybrid type driving device for a vehicle including two shifting mechanisms similar to the above-mentioned ones, and further including an electric motor for engaging with an input shaft of one of the shifting mechanisms. Such hybrid type driving device for the vehicle has three modes of power supply, which are an engine single driving, a motor single driving and a hybrid driving in combination between the engine and the motor. Which mode to be applied is controlled appropriately depending on a driving condition of the vehicle.
And now, since the engine is stopped in the motor single driving state, the engine needs to start in shifting from the motor single driving to the engine driving while the vehicle is driving. Thus, the hybrid type driving device is configured to start the engine (perform cranking) using a rotation of the motor for the vehicle driving or a rotation due to a driving force transmitted from a driving wheel side, and establish an appropriate gear position after starting the engine to perform the engine driving. For this purpose, the hybrid type driving device has a motor start mode and a push-start mode as modes of engine start control. The motor start mode engages the first clutch to start the engine using the driving force of the electric motor during the motor single driving that only uses the motor (electric motor) as the driving source. In a state in which the driving wheels and the engine are stopped, the push-start mode sets each of the gear positions of the first shifting mechanism to neutral and in turn engages the first clutch to start the engine using the driving force transmitted from the driving wheel side.
Incidentally, the above-described push-start mode has a minimum vehicle speed at which the push-start mode is executable. Therefore, if continuing the motor driving at a vehicle speed less than the minimum vehicle speed, the push-start start mode might fail to start the engine, leading to a shortage of state of charge (SOC) of the battery (capacitor). Further, in performing the above-mentioned motor start mode, the hybrid type driving device can transmit no driving force to the driving wheel side due to the first shifting mechanism being neutral. Therefore, when running on a hill-climbing road, the vehicle might move backward under a condition unintended by a driver.
It should be noted that Patent documents 2 to 4 disclose prior arts for performing an engine start control using an electric motor while a hybrid vehicle is running. In order to maintain a stopped state against torque generated by the motor at a time of engine start demand, the hybrid vehicle described in Patent document 2 imparts predetermined braking torque from a braking torque imparting means and in turn generates predetermined torque from the motor after imparting the braking torque. Further, a starting control device for a hybrid vehicle disclosed in Patent document 3 sets a driving system to a neutral position to start an engine using a driving force of a motor. In addition, a control device for a hybrid vehicle disclosed in Patent document 4 starts an engine using inertia torque of the vehicle by fastening a first fastening element at a time of issuing engine start demand during a coast driving in an electric car mode.
However. Patent documents 2 to 4 disclose no art for coping with problems that continuing the motor driving at a vehicle speed less than the minimum vehicle speed of the above-described push-start mode might cause a shortage of state of charge of the battery, and the vehicle might move backward under the condition unintended by the driver in performing the above-described motor start mode.